The field of Nuclear Medicine has utilized radionuclides to produce images to assist in the diagnosis of disease for more than three decades. Commonly, an Anger gamma scintillation camera is utilized to develop an image corresponding to the radionuclide distribution within a patient. The gamma camera generally includes a sodium iodide (NaI) scintillation crystal to detect and convert the photons emitted by a radionuclide within the patient into light pulses, a collimator, interposed between the scintillation crystal and the patient, to permit detection only of photons with the desired orientation, an array of photomultiplier tubes to convert the light pulses produced by the scintillation crystal into electrical signals, a pulse height analyzer to isolate the photopeak corresponding to the isotope within the patient by discarding the electrical signals created by scattered, background and/or other spurious radiation, output devices, such as a cathode ray tube or the like, to provide visual and hard copy images of the output of the pulse height analyzer and a computer for controlling the overall operation of the gamma camera and for analyzing the collected data. Using the Anger camera, gamma rays emitted from the isotopes in the patient's body can be localized in a two dimensional projection and formatted onto standard radiographic film.
Over the last decade, three dimensional image construction using single photon emission computed tomography (SPECT) has become widely used in the field of Nuclear Medicine as a method to enhance spatial and contrast resolution. SPECT uses multiple different projections (usually acquired in a 180 or 360 degree arc around the patient) to allow a computer to reconstruct the three dimensional image of isotope distribution. This information can be viewed using a number of different methods such as tomographic slices along a chosen axis (transaxial, coronal, sagittal, etc.), surface or volume rendering, polar maps or the like.
Unfortunately, as currently practiced, SPECT imaging of radioisotope studies in the human body suffers from several disadvantages. For example, photons emitted from radioisotopes in the organ of interest are attenuated by the body tissue that lies between the organ of interest and the detector-camera. This photon-attenuation varies with each different projection and results in image distortion and artifact.
Another difficulty physicians face when attempting to interpret nuclear medicine images is the lack of anatomic detail in many of the images. Nuclear medicine frequently involves functional imaging in which a physiologic process rather than an anatomical structure is being examined. Common physiologic processes that are investigated include tissue viability, perfusion, bone metabolism, leukocyte distribution and monoclonal antibody binding. Unfortunately, areas of increased radionuclide activity on the scintigraphic images provided by a gamma scintillation camera often indicate the presence of the physiologic process of interest without clear indication of its precise anatomic location.